# Px = P cos θx equation question.

1. Dec 1, 2011

### AcousticBruce

$$P_x = P cos θ_x$$

is Px always the adjacent line of the triangle?

2. Dec 1, 2011

### RTW69

Yes, if the adjacent side is of a right triangle, where P is the hypotenuse and θ is the angle between the Hypotenuse and adjacent side.

3. Dec 1, 2011

### AcousticBruce

Thank you. And a new question.

Why would it be Px for adjacent and on the other side be P without a subscript?

4. Dec 1, 2011

### Staff: Mentor

Presumably a vector of magnitude P is being decomposed into its X and Y components, with θx being the angle between P and the X axis. Thus:

Px = P cos(θx)

and

Py = P sin(θx)

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Last edited: Dec 1, 2011
5. Dec 1, 2011

### RTW69

Your problem tells me that Px is the component of P in the x-direction. The component of P in the y-direction would be labeled Py. P is the hypotenuse. P might be a force at some angle θ from the x-axis and you are trying to find the component of that force in the x-direction which utilizes the Cosine trig function. If you were trying to find component of the force in the y-direction, Py you would use the sine trig function.

6. Dec 1, 2011

### AcousticBruce

So would you ever really have the θy?

7. Dec 1, 2011

### RTW69

Yes, look at gneil's image. θx is measured from the x-axis. θy would be the angle measured from the y-axis and would equal 90 degrees- θx.

8. Dec 1, 2011

### Staff: Mentor

Sure. You never know which angle may be given to you or which one might be determined by some other factor in the problem. Example: problems that specify angles for pendulum strings which reference the vertical direction, not the horizontal.

9. Dec 1, 2011

### AcousticBruce

Thanks a lot guys. Really! I am in a self study of physics and trigonometry. I am studying electronics and it seems that physics and trig would be a good thing to learn. :)